Electricity is indispensable in our daily life. The power outage incurred losses of households and industries. To ensure smooth operation of power network, each process of the power network including power generation, power transmission and power application should be monitored properly.
Within the power network, aging degradation of the cable is one of the important monitored items. Cables are used to carry electric currents and transmit power. Thus they should be durable to various environmental factors such as sun, wind, rain, immersion in seawater, or high-dose radiation in area around nuclear power plants. However, the service life and the insulation the cable provided is significantly decreased under influence of these factors. This poses a serious challenge for maintenance and management of the cable.
In an environment with high temperature, oxidation-resistant material on surface of the cable is gradually degraded; and in a humid environment, void characteristics in insulation materials such as size and density are also changed. As to an environment with radiation, the insulation materials are getting easier to be invaded by water. There is a rapid reduction of the resistance of the insulation materials.
Thus there is an urgent need to provide a method for precise detection of cable aging degradation and further preventing the cable from breaking without any signs and early warning.
Refer to Taiwanese Pat. Pub. No. TW 1237123, a method for detecting degradation of direct current cable is revealed. The method simples the insulation parameter of the direct current cable by a fixed cycle during a sampling period and builds up a trend table by the value sampled so as to judge the variation trend of the insulation parameter of the direct current cable. When the insulation parameter variation is increased dramatically in unit time, the degradation state of the cable is determined as seriously degraded and a warning signal is generated for replacement of the cable.
In this patent, the degradation detection of the cable is by using leakage current of the cable as sample and converting the leakage current into insulation parameter within a period of time. Thus the degradation state of the cable is determined. The method is unable to detect degradation state of samples obtained at different depth in the cable.
Refer to Chinese Pat. App. No. 200810202541.7, a method for testing service life of solid-insulated medium voltage power cable is revealed. Through accelerated water treeing test, 14-day heat/load cycle test, step-up breakdown voltage test at power frequency, step-up breakdown voltage test for measurement of impulse voltage, the statistics of the growth development and the density detection of water tree inside insulation part, electrical tests, and structural detection, a curve showing breakdown voltage test at power frequency changing along with the time of the accelerated water treeing test is obtained. The quality and the service life of the cable can be evaluated according to the curve.
The breakdown voltage test is a common test. However, the breakdown voltage test is carried out on expensive high voltage equipment. Yet the high voltage equipment is not common in general labs.
Refer to CN 201010595007.4, an apparatus for testing aging resistance and water tree resistance of medium voltage power cable is disclosed. The tests are also carried out by breakdown voltage, as the above patent.
Refer to U.S. Pat. No. 7,992,449, a method for assessment of cable strength and residual life is revealed. The method includes steps of random sampling of individual cable wires, mechanically testing the sampled wires, and determining residual life of cable wires according to stress-strain curves. The cable is divided into 8 uniformly spaced sectors, thus forming a typical eight-wedge pattern. The procedure involves inserting and driving the wedges into the wires along a respective wedge line resulting in a plurality of peripheral wedge openings. Instead of the degraded rubber material outside, the sampling target is wire.
Refer to US 20050268734, an electrical condition monitoring method for polymers is disclosed. The method utilizes measurements of electrical resistivity of an age sensor disposed in an electrical cable. The method is also unable to detect aging and degradation of positions at different depths state in the cable.